Load driving circuits are used to drive loads in a variety of applications, such as within polyphase systems to drive electric motors. Many such load driving applications necessitate galvanic or other forms of isolation between the load system and another system delivering power, control and data inputs. Isolation is typically necessitated for a variety of reasons, including safety and reliability concerns associated with higher voltage levels present in the load system.
In one form, a load driving circuit can include a half bridge circuit having a pair of NMOS power transistors connected to the load at respective source and drain terminals to provide corresponding high and low side driving. Using NMOS transistors to provide both high and low side driving can be advantageous due to the typically lower on resistance, higher power capability, and faster switching of NMOS transistors in comparison to PMOS transistors. However, using NMOS transistors to provide high side driving creates the need to provide a functionally isolated or electrically floating supply voltage to a gate driver driving the high side NMOS transistor, as the source of the NMOS transistor is connected to the load.
One difficulty in providing an electrically floating supply voltage to a high side gate driver is that the gate driver draws energy from the source of the supply voltage while driving the load, potentially decreasing the supply voltage to levels that impact the performance of the gate driver itself. Another difficulty is that initializing the electrically floating supply voltage has typically required an undesirably prolonged startup sequence involving the low side gate driver.
Therefore, a need exists for an improved power supply circuit to deliver isolated and/or electrically floating supply voltages to load driving circuits.